Significantly increasing open-circuit voltage of the benzo[1,2-b:4,5-b′]dithiophene-alt-5,8-dithienyl-quinoxaline copolymers based PSCs by appending dioctyloxy chains at 6,7-positions of quinoxaline

Two novel 5,8-disubstituted benzo[1,2-b:4,5-b′]dithiophene-alt-2,3-di(4-methoxylphenyl)-6,7-dioctyloxy-5,8-dithienyl-quinoxaline copolymers of PBDTDT-(Qx-2)-O and PBDTDT(Qx-2)-T were designed and synthesized, in which the substituents are branch dioctyloxy and di(octylthienyl), respectively. Their optical, thermal, electro-chemical and photovoltaic properties were investigated. Compared to the reported analogues, both one- and two-dimensional (1D and 2D) copolymers exhibited outstandingly increasing open-circuit voltages (V_oc) about 0.91-0.95 V in their bulk hetero-junction polymer solar cells (PSCs) with [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) acceptor. The maximum power conversion efficiency (PCE) of 6.31% with V_oc of 0.95 V, short-circuit current of 10.82 mA cm^-2 and fill factor of 61.4% was obtained in the 2D-conjugated polymer of PBDTDT(Qx-2)-T/PC₇₁BM-based device. The PCE value is 1.42 times higher than that of the 1D-conjugated polymer of PBDTD(Qx-2)-O/PC₇₁BM-based device. Our work demonstrates that V_oc values of the quinoxaline-type copolymers based PSCs can remarkably be increased by appending dioctyloxy chains at 6,7-positions of quinoxaline.